Method and Device for Generating and Storing Hydrogen

ABSTRACT

A method and device for charging a metal hydride hydrogen store ( 13 ) is described. It comprises generating gaseous hydrogen in an electrolyser ( 1 ), transporting the gaseous hydrogen to the metal hydride store and feeding the metal hydride store with the hydrogen. For having the hydrogen free of water contamination, the gaseous hydrogen is first transported under a first pressure of at least 1 MPa which pressure is then reduced by a pressure adjusting means ( 11 ) to a second, designated pressure lower to the first pressure, the second pressure being the feeding pressure to the metal hydride store. FIG.  1.

FIELD OF THE INVENTION

The invention relates to a method and a device for generating and storing hydrogen, particularly for providing a hydrogen source for fuel cells, the method comprising the steps of generating gaseous hydrogen in an electrolyser, transporting the gaseous hydrogen to the metal hydride store and feeding the metal hydride store with the hydrogen.

BACKGROUND OF THE INVENTION

Hydrogen storage via a metal hydride, e.g. LaNi or TiZr based alloys with AB2 or AB5 structure respectively is a known method for providing a hydrogen supply for consumers such as fuel cells. Metal hydrides, however, are very sensitive to water in both a liquid and vapour form. Hydrogen generated by electrolysis typically contains fractions of water which must be eliminated before reaching the metal hydride store.

It is known, e.g. from U.S. Pat. No. 5,964,089, FIG. 3, to eliminate the water fraction by a liquid water trap, a condensing coil and a chemical drier. For a complete elimination of any liquid, these elements need to be broadly dimensioned. It is an object of the invention to obtain a nearby complete elimination in a simpler and volume saving way.

SUMMARY OF THE INVENTION

To this end, the method of the invention is characterized in that the gaseous hydrogen is first transported under a first pressure of at least 1 MPa absolute, in the following all pressure data are absolute pressure, which pressure is then reduced by a pressure adjusting means to a second, designated pressure lower to the first pressure, the second pressure being the feeding pressure to the metal hydride store. Further the device of the invention, comprising an electrolyser, a metal hydride hydrogen store and a conduit connecting both these elements, is characterized in that into the conduit a pressure adjusting means is integrated.

The invention is based on the fact that the saturation water vapour partial pressure depends on the temperature but keeps materially constant if the gas pressure of the mixture containing the water vapour is increased. Upon electrolysing, it is possible to generate the hydrogen under a high pressure directly at the electrode by controlling parameters such as voltage, especially if a polymer-electrolyte membrane (PEM) electrolyser is used. A typical electrolysis temperature is 60° C.; then, if the hydrogen is produced under about 0.1 MPa, for 100 molecules of gas, 20 water molecules are present and if the hydrogen is produced under about 1 MPa, for 100 molecules of gas, 2 water molecules are present. On the other hand, the metal hydride hydrogen store at the beginning of charging exhibits a counter pressure of about 0.1 kPa which rises during charging. If the hydrogen-water mixture is fed into the metal hydride store as generated with 0.1 MPa, the metal hydride will soon be spoiled.

The gas mixture under the first pressure is advantageously poor of water molecules, whether generated under the high pressure or afterwards compressed, and under the second pressure which is the counter pressure of the store, still profits of the low content of water molecules.

Additional dehydrating means such as water vapour separators in the form of a chemical absorber can advantageously be added. The time intervals for a replacement of the chemical absorber are large due to the poor total water vapour produced by the electrolyser. Further a liquid water separator such as a condenser working through cooling to ambient temperature or below can be integrated. The danger of system blocking by freezing is negligible due to the low water content as a result of the high first pressure. The liquid water separator contains a gas collecting area and a liquid collecting area, the latter comprising a liquid level sensor controlling a liquid exhaust which contains a pressure reducer and a valve. The collected liquid can be exhausted or recycled to the electrolysis cell or cells. This can guarantee a continuous operation.

According to the invention the pressure adjusting means, in particular a pressure reduction valve, can preferably be a pressure retention valve keeping the pressure at its input side at the first pressure of at least 1 MPa, materially irrespective of the second pressure at its output side.

Further, a first pressure sensitive switch between the electrolyser and the pressure adjusting means is integrated as a safety device to check whether the electrolyser produces a certain minimum pressure; if not, a system failure is assumed and the complete system is shut down. And a second pressure sensitive switch between the pressure adjusting means and the metal hydride store is integrated to switch off the electrolysis upon reaching a certain back pressure of the store that depends on the level of charging.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of the present invention will become apparent from the following description of a preferred embodiment with reference to the accompanying drawings.

FIG. 1 shows the principle of the invention.

FIG. 2 shows a block diagram of one embodiment of the invention.

FIG. 3 shows a section view of a pressure adjusting valve.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

According to FIG. 1, in a stack of PEM electrolysis cells 1, in the drawing for simplicity shown as a single cell, a cathode compartment 2 and an anode compartment 3 are separated by a polymer electrolyte membrane (PEM) 4. To the anode compartment 3, in operation water is added. The compartments contain electrodes to which dc is supplied, whereupon at the anode in compartment 3 oxygen is produced and at the cathode in compartment 2 hydrogen is produced, which is the used product of the electrolysis cells 1. The hydrogen is generated under a pressure of at least 1 MPa, preferably 2.5 to 4 MPa under which pressure hardly any water vapour, escapes with the hydrogen. In this case, hardly any contaminating water will emerge in the cathode compartment 2.

From this compartment 2, a first conduit 5 conducts the hydrogen, nearly without any water content, under the original high pressure to a pressure adjusting device 11, which, in the described example, is a pressure reduction valve. The output of device 11 is connected to a second conduit 12 which leads the hydrogen under a reduced pressure to a metal hydride store 13 where the hydrogen is chemically stored. The metal hydride store 13 develops a counter pressure which is for typically 0.1 MPa at the beginning of charging, and 2 MPa at the end thereof. The difference between the first pressure in the first conduit 5 and the second pressure in the second conduit 12 is maintained by the pressure adjusting device 11 during preferred all stages of charging. Thus, the condition of the hydrogen before reaching device 11 is, preferred constantly, under a higher pressure than needed for the charging and so keeping the water content nearby zero.

If the PEM cell 1 delivers a pressure not sufficient for the delivering hydrogen almost without water, a construction is possible using a compressor in the output conduit 5. After this compressor, the gas stream is moderately dry.

FIG. 2 shows a block diagram of one embodiment of the invention. The electrolysis cell 1 containing the compartments 2 and 3 and the PEM 4 is connected to a water tank 17 via a pump 18 feeding the water to be dissociated under the control of an electronic control device 19 to which the pump is connected via a control line 20. The water is pumped through the anode compartment 3 back to the water tank 17 via a back water line 21 together with the reaction product oxygen. The power supply is managed via a supply unit 22 fed by the control device 19. A stack control line 23 serves for managing the stack of electrolysis cells depicted as cell 1. The water tank 17 has a water supply inlet 25 and a water level sensor 26 which is connected to the electronic control device 19 via a control line 27. Further it has an oxygen outlet 28.

The hydrogen escaping from the cathode compartment 2 through the first conduit 5 and still containing traces of water vapour is first cooled in a radiator 32 to ambient temperature, thereby condensing part of the residual water, and then is led to the water collecting tank 33. The upper part of tank 33 is optionally cooled under ambient temperature, preferably by using Peltier elements. Even a cooling under 0° C. is possible. Due to the low water content as result of the high first pressure the danger of system blocking by freezing is negligible.

In the lower part of tank 33 liquid water is collected and its level is controlled by a level sensor 34 activating an electromagnetic valve 35 when a water level is reached. Between the tank 33 and the valve 35 a pressure reducer 36 is inserted, since in the tank 33 the high pressure of the cathode compartment 2 is still present. The water dispensed from the tank 33 is recycled to the tank 17 via recycling line 37 or purged out to the environment via purging line 38.

The hydrogen leaving the tank 33 via a conduit which is still designated first conduit 5 passes a safety valve 43 which opens e.g. at 0.3 MPa above the first pressure and then enters a chemical absorber 44, known in the art, for finally dehydrating the hydrogen. The hydrogen leaving the chemical absorber 44 passes a pressure sensitive switch 45 which signalizes to the control device 19 if the first pressure is falling below a first threshold e.g. of 2 to 2.5 MPa. If the pressure falls below this threshold after a certain starting period, this is interpreted as a leakage failure of the system and the control device 19 will shut down the system.

The conduit 5 then leads into the pressure adjusting device 11, still under the first pressure of the cathode compartment 3. The pressure adjusting device 11 reduces the pressure to the pressure momentarily demanded by the metal hydride store 13. Several constructions for such device 11 are possible, the preferred example uses a pressure reduction valve, described in FIG. 3 below.

The outlet of the device 11 is the second conduit 12 leading the hydrogen under the second pressure to the metal hydride store 13. A pressure sensitive switch 48 checks the pressure in conduit 12 for finding out the end of the charging step, signalizing this to the control device 19 and then switching off the system. As mentioned, the back pressure of the store 13 increases while charging goes on and also is dependent on the temperature of the store. For this reason, after a first switch off, the temperature will decrease lowering also the back pressure, which has the consequence that the switch 48 causes a restart of the system, which sequence may occur several times.

Into the second conduit 12 according to the described example a conduit flushing valve 49 is inserted to be used for a controlled flushing after an extended non-use. The metal hydride store 13 is detachably connected to the conduit 12 for being exchanged after complete charging. A mounting sensor 50 checks the correct assembly of the metal hydride store 13 to the conduit 12, and signalizes it to the control device 19.

The construction of FIG. 3 is a basic type of a back-pressure valve. The conduit 5 is connected to a hydrogen input 55 and the conduit 12 is connected to the hydrogen output 56. These ports 55 and 56 are situated within a cylinder block 57 wherein a piston 58 is slidably disposed in a bore and is pressed by a helical spring 59 until a seal 60 in the front surface of the piston 58 impinges on raised-faced flange 65 communicating via a duct 66 with the output 56 while the space surrounding the flange 65 and limited by the block 57 and the piston 58 communicates via a duct 67 with the input 55. Thereby, between the flange 65 and the seal 60 a circular ring shaped sealing area 68 is formed. The opposite surface of the piston 58 communicates via a hole 69 with the surroundings thus atmospheric pressure is applied to the backside of the piston. Between the ducts 66 and 67, the pressure difference between the higher first pressure and the lower second pressure exists, both pressures together shifting the piston 58 against the force of the spring 59, thereby opening a gap between the elements 60 and 65 which acts as a choke through which the hydrogen passes to the conduit 12 under the second pressure. The second pressure is determined by the metal hydride store 13 and its charging status.

REFERENCE LIST

-   1 PEM electrolysis cell -   2 cathode compartment -   3 anode compartment -   4 polymer electrolyte membrane (PEM) -   5 first conduit -   11 pressure adjusting device -   12 second conduit -   13 metal hydride store -   17 water tank -   18 pump -   19 electronic control device -   20 control line -   21 back water line -   22 supply unit -   23 stack control line -   25 water supply inlet -   26 water level sensor -   27 control line -   28 oxygen outlet -   32 radiator -   33 water collecting tank -   34 level sensor -   35 valve -   36 pressure reducer -   37 recycling line -   38 purging line -   43 safety valve -   44 chemical absorber -   45 pressure sensitive switch -   48 pressure sensitive switch -   49 conduit flushing valve -   50 mounting sensor -   55 hydrogen input -   56 hydrogen output -   57 cylinder block -   58 piston -   59 spring -   60 seal -   65 raised-face flansh -   66 duct -   67 duct -   68 sealing area -   69 hole 

1-11. (canceled)
 12. A method for charging a metal hydride hydrogen store (13), the method comprising: generating gaseous hydrogen in an electrolyser (1); transporting the gaseous hydrogen to the metal hydride store at a first pressure of at least 1 MPa; providing a pressure adjusting means (11) for reducing the first pressure; reducing the first pressure to a second pressure lower than the first pressure by use of the pressure adjusting means; and feeding the metal hydride store (13) with the gaseous hydrogen at the second pressure.
 13. The method of claim 12 wherein the first pressure of the gaseous hydrogen is generated in the electrolyser (1).
 14. The method of claim 12 further comprising prior to reducing: further providing dehydrating means (32, 33, 34) for dehydrating the gaseous hydrogen; and dehydrating the gaseous hydrogen by conducting it through the dehydrating means.
 15. A device for generating and storing hydrogen gas comprising: an electrolyser (1); a metal hydride hydrogen store (13); a conduit (5, 12) connecting said electrolyser to said metal hydride hydrogen store; and a pressure adjusting device (11) which is integrated into said conduit.
 16. The device of claim 15 wherein the electrolyser (1) incorporates a polymer electrolyte membrane PEM (4).
 17. The device of claim 15 wherein said pressure adjusting device (11) is a pressure retention valve having an input side (55) and an output side (56) which maintains the pressure at the input side (55) at a first pressure of at least 1 MPa regardless of the second pressure at the output side (56).
 18. The device of claim 15 wherein said conduit is further comprised of a first conduit (5) and a second conduit (12).
 19. The device of claim 18 further comprising liquid separator means connected to said first conduit for separating water from the hydrogen gas.
 20. The device of claim 19 wherein said liquid separator means further comprises: a radiator (32) connected to said first conduit and a water collecting tank (33) connected to said radiator.
 21. The device of claim 20 further comprising: a liquid level sensor (34) connected to said water collecting tank; a valve (35) connected to and controlled by said liquid level sensor; and a pressure reducer (36) connected to said valve and to said water collecting tank and controlled by said liquid level sensor.
 22. The device of claim 20 further comprising a water vapour separator which functions as a chemical absorber (44), said separator being connected to said water collecting tank.
 23. The device of claim 22 further comprising a safety valve connected between said water collecting tank and said water vapour separator.
 24. The device of claim 22 further comprising a pressure sensitive switch means (45) connected between said water vapour separator and said pressure adjusting device for determining whether the first pressure has fallen below a first threshold.
 25. The device of claim 18 further comprising a pressure sensitive switch means (48) connected to said second conduit between said pressure adjusting device and said metal hydride store for determining whether the second pressure is above a second threshold.
 26. A device for generating and storing hydrogen gas comprising: an electronic control device (19); a supply unit (22) connected to said electronic control device; a PEM electrolysis cell (1) having a cathode compartment (2) and an anode compartment (3), the cathode compartment of which is connected to said supply unit; a water tank (17) having a water supply inlet (25), a water level sensor (26) connected by a control line (27) to said electronic control device, an oxygen outlet (28), a back water line (21) connected to the anode compartment of said cell and a recycling line (37); a pump (18) controlled by said electronic control device, said pump being connected to said water tank and to the anode compartment of said cell; a first conduit (5) connected to the anode compartment of said cell; a radiator having an input and an output, being connected at its input to said first conduit; a water collecting tank (33) having an input and two outputs, said water collecting tank being connected at its input to the output of said radiator; a pressure reducer (36) connected to the first output of said water tank; a first valve (35) connected to said pressure reducer; a recycling line connected on one end to said first valve and on the other end to said water tank; a purging line connect to said recycling line and to said first valve; a liquid level sensor (34) located inside said water collecting tank and electronically connected to said first valve; a safety valve (43) connected to the second output of said water collecting tank; a chemical absorber (44) having an input and an output, connected at its input to the second output of said water collecting tank and to said safety valve; a first pressure sensitive switch (45) connected to the output of said chemical absorber and controlled by said electronic control device; a pressure adjusting device (11) having an input and an output connected at its input to the output of said water collecting tank and to said pressure sensitive switch (45); a second conduit (12) connected to the output of said pressure adjusting device; a second pressure sensitive switch (48) connected to said second conduit and controlled by said electronic control device; a conduit flushing valve (49) having an input and an output connected at its input to said second conduit; a mounting sensor (5) connected to the output of said conduit flushing valve and controlled by said electronic control device; and a metal hydride store (13) connected to the output of said conduit flushing valve. 